The overall goal is to define the pathophysiology and etiology of various clinical types of age-related osteoporosis. Patients, both age-matched and younger controls, will be examined by a wide variety of methods including calcium balance, radiocalcium kinetics, bone histomorphometry after double tetracycline-labeling, bone densitometry at peripheral and central sites, and measurement of the major calciotropic hormones (PTH, calcitonin and calcitriol), and of both old and some newly-developed biochemical markers of bone remodeling, including urinary hydroxyproline, and serum osteocalcin, osteonectin and bone-specific alkaline phosphatase. Aim 1 is to characterize bone turnover and its regulatory factors in type I osteoporosis, by four protocols. Aim 2 is to identify reasons for individual susceptibility to type I osteoporosis. Aim 3 is to identify the cause of abnormal calcium metabolism in type II osteoporosis. Aim 4 is to elucidate the mechanism of increased parathyroid function in type II osteoporosis.